Since exothermic elements such as power transistor, thyristor, rectifier, transformer, power MOS FET and the like suffer from deterioration of their characteristics or, in the worst case, break down due to their generating heat, a means of conducting heat, such as fitting them with a heat sink or mounting them on a metallic chassis, has so far been taken in installing them. Therein, a sheet-form thermal conductive electric insulating material has generally been laid between an exothermic element and a heat sink with the intention of heightening electric insulating capacity and thermal conductive efficiency.
As for the thermally conductive electric insulating material as described above, silicone rubber compounded with beryllium oxide, aluminum oxide, aluminum hydroxide, magnesium oxide, zinc oxide or the like has been known (for the details of which Japanese Kokai Koho (Unexamined Publication) No. 47-342400 can be referred to).
However, thermally conductivity of such a thermal conductive, electrical insulating material as cited above was 4'10.sup.-3 cal/cm.multidot.sec .degree.C. at the highest, and filling such a material with a large quantity of thermal conductivity-providing powder for improvement on the thermal conductivity caused serious deterioration in moldability.
On the other hand, using boron nitride, which is excellent in thermal conductivity, made it feasible to increase the thermal conductivity of compounded silicone rubber up to 5.times.10.sup.-3 cal/cm.multidot.sec .multidot..degree.C. or higher. However, the silicone rubber compounded with boron nitride was low in strength under uncured condition (green strength), so that it was poor in moldability. Even after it was cured, the strength thereof was so low that reinforcement with glass cloth or the like was required. Accordingly, it was difficult to make moldings of any other form than that of sheet. Moreover, it had a disadvantage that the ratio of raw material cost to production cost was high due to expensiveness of boron nitride.
As another thermally conductive electric insulating material, silicone rubber filled with a large quantity of spherical aluminum oxide powders has been known (for the details of which Japanese Kokoku Koho (Examined Publication) No. 58-22055 and Japanese Kokai Koho No. 64-69661 can be referred to). However, on the occasion that aluminum oxide powders were compounded in an amount of 500 parts by weight or more, the moldability of the resulting silicone rubber was lowered even when aluminum oxide powders differing in size distribution were used in combination. Under such a condition, it was difficult in particular to perform calendering and extrusion molding.
With the recent advance of miniaturization of various kinds of apparatuses, the number of elements covered with an insulating material has been increased, and in proportion thereto it has acquired importance to shorten an insulating creeping distance of exothermic elements. Therein, tubular or case-form insulating thermally conductive moldings have been used, so that moldability of thermally conductive silicone rubber compounds has been an important point.
Under these circumstances, we have pursued intensive studies of thermally conductive, electric insulating materials excellent in moldability. As a result, it has been found that silicone rubber can be filled with 500 parts by weight or more of a spherical aluminum oxide powder when the combination of an organopolysiloxane having a high degree of polymerization and that having a low degree of polymerization is used as the basis to be compounded and thereby it becomes feasible to obtain a thermal conductive silicone rubber composition having thermal conductivity of 5.times.10.sup.-3 cal/cm.multidot.sec.multidot..degree.C. or higher and excellent moldability, thus achieving the present invention.